Brake pedal constantly going soft

[rebel.ranter]

In the M6 the brake pedal has gone soft & I am stumped as to why this is happening.

1. The first time it did it was just after I bought it, the pedal was soft & it failed the NCT for excessive brake pedal travel. A brake bleed was completed & it was fine for about two weeks.

2. Then it went soft again, about two weeks after the brake bleed was completed, again with pedal getting soft & the pedal travel is ridiculous. It was diagnosed as a defective master cylinder. The master cylinder was replaced and everything was good....for about two weeks.

3. So after two weeks with a new master cylinder it then was left in for another brake bleed & without any driving (it was in the dealer's compound) the pedal again went soft after a brake bleed.

4. This time the issue was diagnosed as a defective Servo. So the servo was replaced.

Here were are again about 2 weeks after that & the fucking pedal is soft again!

Anybody got any ideas as to why this in happening? The dealer doesn't seem to be doing a great job so far at identifying root cause but maybe it is just one of those hard to trace issues. The car is under warranty, while it is good that I am not being hit in the pocket for the work I would really rather just have the car fixed & be able to drive it. It's spent nearly 25% of my ownership in the dealers, brakes are not rocket science stuff either so the whole thing is getting frustrating.

Any help appreciated.

After each "fix" the pedal feels great for a week or so, and there's no leaks. It is wrecking my head at the minute.

[Smokey Bear]

Is the fluid level changing at all? I presume you've checked it but I said I'd put it out there anyway. It sounds like it's getting air into the system somehow although the fact that it's not been driven half puts that theory out the window.

Definitely a strange one. I'd also go about checking all the brake hose and calipers for leaks, could omit be possible that a brake hose was caught off the ground somewhere? You could isolate the calipers by putting a vise grips on each brake line and leave the car for a while to see if the softness returns.

Is there any leaks around the abs pump?

[Venom]

I'd agree. Sounds like air in the system or seals gone. Had a similar issue with an audi. Was going to the floor until all 4 corners were bled.

[Valdelisbmw]

Is the fluid level changing at all? I presume you've checked it but I said I'd put it out there anyway. It sounds like it's getting air into the system somehow although the fact that it's not been driven half puts that theory out the window.

Definitely a strange one. I'd also go about checking all the brake hose and calipers for leaks, could omit be possible that a brake hose was caught off the ground somewhere? You could isolate the calipers by putting a vise grips on each brake line and leave the car for a while to see if the softness returns.

Is there any leaks around the abs pump?

If it would be leaking !!! U would lost all brake fluid in minutes ! It wouldn't be ok for days weeks or hours .

[rebel.ranter]

Yeah, there are no leaks. That was checked many times. Here's a few things that I was thinking of but they are kind of "grasping at straws" type ideas.

- Would a hairline crack in the master cylinder fluid reservoir cap allow air in over time (2 weeks) & cause the pedal to go soft?

- Am I potentially braking too hard boiling the fluid? Would this cause the fluid to go off? I'm not that hard on the brakes, at least I think so anyway.

- The pedal stays good for about a week or two,so am I correct in saying that the flexible brake lines & pads/discs should not be to blame?

:@ Very frustrated.

Thanks for the replies so far.

[Hobbs]

Unlikely you'd be boiling the fluid. Unless youre really trying on the back roads?!:hehe!:

Does the pedal go hard if you give it a few pumps or stay soft? If it goes hard after one or two pumps it could be something simple like a pad binding? Ie a pad getting slightly stuck on the carrier and needs an extra push to get to the disc.

[rebel.ranter]

Unlikely you'd be boiling the fluid. Unless youre really trying on the back roads?!:hehe!:

Does the pedal go hard if you give it a few pumps or stay soft? If it goes hard after one or two pumps it could be something simple like a pad binding? Ie a pad getting slightly stuck on the carrier and needs an extra push to get to the disc.

Thanks I just thought I'd check, as you know there's a fair bit of muscle under the bonnet and I thought that maybe I was putting it under a it more pressure than normal. Go faster, stop faster scenario.

Why would the pads only start to bind after a week or so & then instantly recover after a brake bleed? To answer your question yes the pressure does get better with pumping,but is that always the case, e.g. even if there was air in the system?

[HealyC]

I know you know this Mark but I'm sure they done the active brake bleed procedure?

[rebel.ranter]

I know you know this Mark but I'm sure they done the active brake bleed procedure?

Christ, I hope they have done it! Otherwise a lot of money has been thrown away on master cylinders & servos!

I am tempted to do a sneaky bleed of the system myself, but would be afraid it would void my warranty. At the same time its a joke that it has to go in for a 4th time to troubleshoot, particularly when it was in for a 3 week stint the last time.

[HealyC]

I know you know this Mark but I'm sure they done the active brake bleed procedure?

Christ, I hope they have done it! Otherwise a lot of money has been thrown away on master cylinders & servos!

I am tempted to do a sneaky bleed of the system myself, but would be afraid it would void my warranty. At the same time its a joke that it has to go in for a 4th time to troubleshoot, particularly when it was in for a 3 week stint the last time.

Had a similar issue before with an X5 and it turned out to be pads. The rear pads weren't sitting in the brake caliper carrier correctly so the piston was travelling too much to squeeze the pads. Fitted them correctly and it was perfect.